There are approximately 100,000 pacemakers implanted annually, about 10% are of atrial type. One of the major limitations of this type of pacer is the lack of suitable lead. Even so, atrial and atrio-ventricular pacers are being increasingly recommended, and used.
Ventricular stimulating pacemakers are the preferred method of treatment for atrio-ventricular conduction defects, however, 20% of these pacemakers are implanted in patients with sino-atrial nodal disease. The reason that the ventricular pacemakers are used rather than atrial stimulating pacemakers is mainly related to the difficulty in implanting and lack of stability of atrial leads and the resultant poor reliability of P-wave sensing and atrial pacing. By using ventricular stimulating pacemakers, the patient with a normal atrio-ventricular conduction system is deprived of the hemodynamic advantages of atrial systole. This impared function reduces the patient's exercise tolerance and may result in congestive failure in those patients with poor ventricular function.
The advantages of atrial pacemaking are real for both tachycardia and bradycardia syndromes. At present, several basic types of atrial pacemakers are available. One type can be used to pace and sense in atria only. This is merely a modified ventricular pacer. Another senses the P-wave in the atria and then, following an appropriate delay may stimulate the ventricule to pace if the A-V conduction is blocked. Still another type pacer has the ability to sense and pace in both the atria and ventricle. The last two types of pacers may be used in patients with complete A-V block while the last type may be used with S-A nodal bradycardia A-V and/or tachycardia syndromes.
Conventional transvenous ventricular lead systems will not remain in place in the atria, therefore, an atrial lead must be either sutured to the atria directly or a special "J" shape atrial lead may be passed transvenously. It is this latter J-shaped lead system either in unipolar or bipolar configurations which are preferred mainly because of its transvenous placement.
Common forms of the J-lead electrode are disclosed in U.S. Pat. No. 3,939,843 Smyth, and U.S. Pat. No. 3,890,977 Wilson.
The disadvantages of the J-lead include a large diameter (.gtoreq.8 Fr) French catheter diameter especially at the radius bend required to maintain the curve; the difficulty in placing such a large stiff lead; and a high dislodgement rate as compared to ventricular leads. In addition, the tip of the J-lead can only be positioned in one place and that is in the atrial appendage, thus negating the ability to position the electrode for minimum pacing threshold and maximum P-wave sensing.
Others have resorted to the use of one or more pliant tines which extend from the electrode tip at an acute angle as "anchor" means to maintain the electrode tip in position. Exemplary of this form of electrode is the electrode disclosed in U.S. Pat. No. 3,902,501 Citron et al.
Flexing of implanted electrodes has also been a problem as such an electrode must flex in the order of 30 million times each year. The Chardack U.S. Pat. No. 3,348,548 and the Lajos U.S. Pat. No. 4,057,067 discloses an electrode wherein the electrical conductor is in the form of spring coils coated with electrically insulating and body compatable material which has now become a fairly uniform procedure for constructing the electrode's conductors.
It is also known to form a cardiac pacer lead which is formable, at the time of pacer implantation by heating the pacer lead as disclosed in the O'Neill U.S. Pat. No. 4,154,247.